November 26, 2007

Cryptomundians Talk Hybrids

Here are some recent things said by Brent Swancer (mystery man) and Sheila Collins (kittenz) in an exchange about hybrids, which was going off-topic within another blog.

The opinions expressed in the following comments do not (necessarily) express those of any Cryptomundo bloggers, and should be viewed as readers’ commentary versus verified factual information that has been fully researched, although it may have been.

If people wish to discuss hybrids, here it is the discussion originally posted as comments only.

Brent Swancer on hybrids:

Not all hybrids get huge, and there are hybrids that are smaller than both parents. For example, whereas you have ligers (the offspring of male lion and a tigress) that get spectacularly huge, there are also “tigons” which are born of a male tiger and a lioness. These tigons are smaller than both parents and are incidentally a lot rarer because of the apparent increased difficulty of getting a male tiger to mate with a female lion. there is an interesting biological reason for this, but I won’t ramble too much here.

Yes, you are right. It was three hybrid offspring born in Washington DC. I remember reading about the story of Willy. While we are on bears, it is important to note that while this article describes the first officially documented natural grizzly/polar bear hybrid, there have been other unconfirmed stories of this happening. A grizzly taxonomist by the name of C Hart Merriam allegedly found a bear that was quite different than other grizzlies and he thought he had found a living fossil of some kind, even giving it the scientific name of Vetularctos inopinatus (”ancient, unexpected bear”). He thought it was from a seperate lineage from Ursinae. It was apparently killed in 1864 in Canada. Nowadays it is thought that this was either a grizzly/polar bear hybrid, or an unusual specimen of grizzly. There was another one reportedly shot on Kodiak island that was an enormous, off white bear with hair on its paws. This paw hair suggests it was not a Kodiak bear but rather it was either a natural hybrid or an aberrant polar bear. I hope I am remembering these names, places and dates accurately.

You are absolutely correct in that there have been other hybrids too. There have successful matings of brown and grizzly bears as well as brown and polar bears. I agree with you, I am not surprised this happens. Most species of bear (except two) can crossbreed, and most have a set of 74 chromosones. One exception is the spectacled bear, with 52 chromosones. The other species that differs from most bears in number of chromosomes is the giant panda, with 42. What you said about the relationship with brown bears and polar bears is also very true. Some DNA studies have shown that some brown bears are actually closer to polar bears than to other brown bears, begging the question of whether they are even technically two different species.

Don’t forget servicals and caravals, which are hybrids between serval and caracal parents. These apparently are sometimes bred for the home market, but wind up being unruly pets due to their territorial nature and incredible jumping abilities. I think there was also a one time only successful crossbreeding of a tiger with a leopardess, which resulted in what was called a “tigard”. In India , in the early 1900s I believe, there were reports of what was called a “dogia”, which is a cross between a leopard and tigress. There were sightings of very large leopards with abdominal striping, but unfortunately it seems these were never confirmed although one was apparently shot. I’ve even heard of crosses between lions and leopards (liards) as well as hybrids between blackfooted cats and domestic cats. I’m sure I’m forgetting some. Cats aren’t really my area, and seeing as you know your cats, I’m sure you would know better than me.

Concerning hybrid vigor, there is an interesting hypotheses that is sometimes used to explain the growth dysplasia in some big cat hybrids, namely with ligers and tigons. If I remember correctly, the idea is that a lion’s competitive mating strategy, a lion’s genes favor larger offspring. This is due to the social need to defend a pride and maintain status in a pride often containing several males. Size has a lot of advantages. However, since a lioness gives birth to up to 5 cubs, the female transmits a growth inhibiting gene to balance out the size favored by the male lion’s genes. Tigers, on the other hand do not have the same pressure to fight for status and lack the genetic disposition to produce larger and larger competing offspring. Add to that the fact that a tigress gives birth to fewer cubs, and there is no need for a growth inhibitor.

So basically with a lion father and tigress mother, you have offspring that grow unfettered due to being denied a growth inhibitor. With tiger fathers and lion mothers, you have no genes cueing for extreme size, and yet the growth inhibitor takes effect, limiting the tigon’s size. I believe this is only a hypothesis, with other competing hypotheses out there as well, but it is fascinating nonetheless. There are many reasons for hybrid vigor, depending on the species involved, but this is one example I find interesting.

I agree that hybridization can be ok under experimental circumstances and if the animal is treated well. The problem though is that sometimes hybrids will display health problems, being prone to cancer and other illnesses, and often have a shorter lifespan. Apparently depending on the species they can also have behavioral, and or psychological problems due to competing instincts from two parents with very different social structures. Therefore, I am not sure how humane it is to produce hybrids between certain species. Anyway, I find hybridization fascinating myself and enjoy everyone’s comments here.

Some other thoughts on what you said. I also believe that hybridization plays an important evolutionary role although traditionally it hasn’t always been seen that way. In my opinion hybridization can create genetic diversification. Simply put, a hybrid genotype that causes increased fitness in a given habitat is conveying the same sort of selective advantage that would be caused by genetic mutation.

Hybridization is not always a positive thing, though. While natural hybridization can increase diversity, the problem comes when genetic hybridization is influenced by human activities, specifically introduced species and fragmentation of habitats. Especially with introduced species, the genetic integrity of a native species can be compromised, sometimes even to the point of extinction. There are many examples of native animals decreasing in numbers or disappearing due to interbreeding with the sometimes more readily available or aggressive introduced species, so basically you have the native species being sort of “absorbed” into the non native one, with blurring of genetic integrity as a result.

With non localized hybridization, you can get what is referred to as a “hybrid swarm”. This is basically a population or group of populations made up entirely of hybrids by varying generations, with a lot of mating between hybrids. Especially in this case, you get that genetic blurring, called “genetic introgression” caused by the flow of genes between the hybridized populations which can originate from 2 or more parent taxa. Depending on a number of factors, this can lead to a swamping of one species by the “hybrid swarm”.

An outcome of hybridization that can definitely lead to speciation is what is called a “hybrid taxon”. In this case you get a stable population of individuals with a set of traits distinct from the parent taxa. These populations are independently evolving and one could say that for all intents and purposes, they either comprise a new species or are fast on their way to becoming one.

This is all very fascinating for me, since one of my areas is the effect of introduced animals on native ecosystems, which often involves interbreeding between native and non native species. Hybridization in this case can cause genetic diversity, but can also cause problems and sometimes it is hard to see which way it will go for quite some time. As a basic rule, I think occasional hybridization between two naturally occurring populations is generally good, hybridization between native and introduced species not so much.

I thought I may as well list some examples of introduced species threatening a native species through hybridization and introgression so you can see what I mean.

One is the Sitka deer (cerus nippon), a Japanese species that was introduced to Great Britain over a hundred years ago. This introduced deer has interbred with the native red deer (Cervus elephaus) to the point that the genetic integrity and fitness of the latter has been compromised.

Then there is the case of the Ethiopian wolf (Canis simensis) and domestic dog. Since the domestic dog is much more common, and have been witnessed mating with the wolves we have a situation where a rarer species’ mating with a common introduced species could threaten the former. There is already evidence of introgression present with the Ethiopian wolf.

The domestic cat swamped the European wild cat (F. sylvestris) as well as the African wildcat (F. lybica). This sort of thing seems to happen a lot with especially carnivores. The same happens with birds, fish, amphibians, insects, and plants. Fish in particular have shown spectacular examples of hybridization wiping out native species, one reason being that fish hybrids are often fertile. My point is to illustrate that hybrids are not some sort of magic pill for species diversification and their effect is not always a positive one.

The list goes on and on. So while I think hybridization and hybrid vigor can be beneficial and can play a role in speciation and increased fitness of species, it sadly is not always the case.

Sheila Collins on hybrids:

I have often seen this particular bear referred to as “the world’s first recorded polar bear-grizzly bear hybrid”. It should be referred to as “the world’s first documented wild polar bear/grizzly bear hybrid”. Test matings in zoos, as well as some accidental matings, have occurred and the hybrid offspring are as viable as the purebred offspring of either species. That is not too surprising, since it’s thought that the polar bear diverged from the brown bear very recently, and grizzly bears are considered to be conspecific with brown bears.

Other subspecies of brown bears are known to have produced hybrid offspring with polar bears in captivity too. A male polar bear accidentally got into an enclosure with a female Kodiak bear at the National Zoo in Washington, DC. They had (I believe), three cubs. One of the hybrids was named Willy. He was very similar to this bear in appearance. He lived at the zoo for several years. He grew to be HUGE. Maybe he was an instance of the type of “growth dysplaysia” that occurs in male ligers; I do not know and I have not seen any documentation of that. Often hybrid animals grow to be larger than animals of either parent species. Apparently, though, test matings were carried out between some of the hybrid offspring, and they were able to breed successfully with each other, producing living cubs, so in that way they were unlike most ligers, the males of which are usually sterile. That’s another indication that polar bears and brown bears are closely related.

It’s really odd too, in that some interspecies hybrids differ depending on which species is which parent. Tigons certainly come first to my mind, but also, for instance, the hinny – the offspring of a stallion and a jenny (female donkey) – is, as a rule, smaller, less robust, and has less strength and stamina than its counterpart, the mule, which is the offspring of a jack donkey and a mare. Male mules and ligers are usually sterile too, although many females are fertile. Tigons and hinnies apparently do not have such a high percentage of sterile adults, although I have not seen concrete documentation of that, just anecdotal reports. Tigons and hinnies are said to be less tractable, more nervous and more aggressive than ligers and mules, too. Female tigons and ligers, when fertile, can mate with either parent species. Horse/zebra hybrids, and hybrids of horses and wild asses or Przewalski’s horse, are usually robust animals, evidence that the entire modern genus Equus are pretty closely related to one another.Lion/leopard hybrids that I have seen, where the leopard is the male parent, also seem to have hybrid vigor, although they do not attain the size of ligers. I am not aware of any lion/leopard hybrids where the male parent is the lion. My guess is that the size difference between the two species is so pronounced that it might not be possible for a female leopard to carry a litter of such large offspring to term.

I’ve seen photos of puma/leopard hybrids too. They are lovely creatures, and everything I can find on them says that they matured at about half the size of the parents, but I do not know which parent was which species.

I’ve also seen photos and descriptions of ocelot/puma hybrids, where the ocelot was the male parent. They were gracile cats which looked exactly as one would expect: a bit larger than the ocelot, quite a bit smaller than the puma. Apparently no hybrid vigor there, because most of the cubs that have been born have either been stillborn or die when very young. Only a few have lived to maturity, and I have seen no information as to whether they were fertile or sterile.

Then there are natural lynx/bobcat hybrids, which are occurring in the wild in some parts of the cats’ ranges. They are intermediate between the two species in size and appearance, and apparently get on as well in the wild as either parent.

The subject of interspecies hybridization is a fscinating one. I’m opposed to breeding ligers and other hybrids of wild animals for entertainment’s sake, simply because so many of the animals end up in terrible circumstances. But I have nothing against hybrid test matings, so long as they are carried out for research purposes and the animals are treated humanely for life. There are some things that can be learned no other way. Who would have dreamed that there would be such a difference in ligers and tigons, for instance? The differences are fascinating, and studying them sheds light on the relationship between species and gives us clues as to how some now-exticnt species may have looked behaved.

I believe that there is more hybridization in the wild than is generally recognized, and that it is one avenue that sometimes, albeit very infrequently, can lead to speciation.

Certainly hybridization in the wild has been a problem for people who are trying to save the Red Wolf from extinction. They hybridize so readily that it was a fight just to get the Red Wolf recognized as a distinct species.

On the other hand, the Florida Panther may have been saved when animals from the Texas subspecies of cougar were brought into the population to provide relief from inbreeding. Of course, that was not really interspecies hybridization, but intraspecies hybridization, sinch the breeding occurred between subspecies.

Some modern species, especially those that appear to be very closely related, may indeed have experienced hybridization at some point in their evolution. We’ll probably never know for sure, but the idea is plausible.

At some point during speciation, the evolving species may, from time to time, encounter the species from which it is diverging, and they may mate and produce viable offspring which in turn live long enough to reproduce. I believe that this is what happened with polar bears as they were diverging from brown bears, and from the reports of wild hybrids that turn up from time to time, it still happens on occasion.

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